1. Field of the Invention
The present invention generally relates to an electromagnetic servo device. More particularly, the invention relates to an electromagnetic servo device for electric power steering systems for vehicles.
2. Description of Relevant Art
In view of problems on the hydraulic type power steering system for vehicles, such as that the structure thereof was complicated and that, due to the necessity of constantly operating a hydraulic pump, the power loss was relatively large, in recent years there have proposed a variety of electric type power steering systems for vehicles.
In those electric type power steering systems were employed various types of electromagnetic servo device.
Those types of electromagnetic servo device included an input shaft adapted to be operatively connected to a steering wheel, an output shaft adapted to be operatively connected, through a steering gearbox or otherwise, to a tie rod of a road wheel to be steered, an electric motor for supplying the output shaft with auxiliary torque, a torque detection mechanism for detecting the magnitude as well as the direction of steering torque acting on the input shaft, and a driving control circuit for sending to the electric motor an armature current of such a magnitude and in such a direction as necessary in accordance with a detection signal from the torque detection mechanism.
By the working of such servo device, the electric type power steering systems had adequately lightened steering forces needed to be exerted for operating the steering wheel, thus achieving favorable steering characteristics.
In this respect, those power steering systems had accompanied desiderata in relation to preference for rapid application of relatively large auxiliary torque to the output shaft of servo device at low vehicle speeds as well as for controlled application of auxiliary torque thereto at middle and high vehicle speeds; the preference arising from the fact that, in general, the necessary force to be exerted for operating a steering wheel is relatively large at low vehicle speeds, but not so large at middle and high vehicle speeds.
To achieve such desideratum, there has been disclosed an electric power steering system for vehicles in Japanese Patent Application Lay-Open Print No. 50-38228, laid open on Apr. 9, 1975, in which an electromagnetic servo device included an electric motor having an armature current controlled to be sent thereto in accordance with the vehicle speed.
In this Japanese Patent Application Lay-Open Print, the ratio (Io/Ti) of the armature current (Io) to steering torque (Ti), that is, the ratio of the armature current to torque acting on an input shaft of the electromagnetic servo device, was controlled to be decreased as the vehicle speed was increased. More particularly, the above ratio (Io/Ti) was controlled to be substantially continuously changed from a certain value to zero, as the vehicle speed was varied from a certain low speed to a predetermined high speed, whereas the input shaft of the servo device was adapted to be directly coupled with an output shaft thereof when the ratio (Io/Ti) was reduced to zero.
Thus, in the electric power steering system according to the aforesaid Japanese Patent Application Lay-Open Print, between the torque (Ti) acting on the input shaft of the electromagnetic servo device and output torque (To) from the output shaft thereof, there were found such relations as shown in FIG. 7 of the accompanying drawings. FIG. 7 is a graph representing an input-output characteristic of the above electric power steering system.
In FIG. 7, the axis of abscissa represents the torque Ti acting on the input shaft, and the axis of ordinate, the output torque To of the output shaft. Designated at reference character Do is the dead zone of a torque detection mechanism in the electromagnetic servo device.
Now, for the aid of comprehension, description will be made below only of a region extending to the right of an origin O of the graph of FIG. 7, that is, of the characteristic relations of the steering torque Ti to the output torque To, when the former is acting in a clockwise direction about the input shaft. Those relations depicted in the left half region of the graph are analogous to those depicted in the above-defined right half region, and will be easily understood without particular description.
While the vehicle speed was kept at a given speed in a low speed region, as the steering torque Ti acting on the input shaft was increased from zero, the output torque To increased, in the interior of the dead zone Do, along a line segment Th straightly extending from the origin 0 with a certain positive gradient and, in the exterior of the dead zone Do, along another line segment Tl straightly extending, from the cross point between the line segment Th and the boundary of the dead zone Do, with a larger gradient than the gradient of the line segment Th.
While the vehicle speed remained at a given speed in a middle speed region between the low speed region and a high speed region, as the steering torque Ti was increased from zero, the output torque To increased, in the interior of the dead zone Do, again along the line segment Th and, in the exterior of the dead zone Do, along still another line segment Tm straightly extending from the aforesaid cross point with a middle gradient between the respective gradients of the line segments Th and Tl. This is because the ratio Io/Ti of the armature current Io to the steering torque Ti was decreased, in accordance with the vehicle speed change from the low speed to the middle speed, as already described.
On the other hand, while the vehicle speed was maintained at a given high speed equal to or higher than the aforesaid predetermined high speed, as the steering torque Ti was increased from zero, the output torque To increased, in both the interior and the exterior of the dead zone Do, along the line segment Th, thus simply corresponding to the steering torque Ti. This is because, at those vehicle speeds equal to or higher than the predetermined high speed, the armature current Io was controlled to be zero and the input shaft was directly coupled with the output shaft, as already described.
With such input-output characteristic, the electric power steering system according to the aforesaid Japanese Patent Application Lay-Open Print achieved a favorable control of the output torque to be varied in accordance with the vehicle speed.
As is observed in general cases, in this power steering system, at given speeds in the low, middle, and high speed regions, the output torque To from the output shaft had proper ranges of values thereof, respectively, of which maximum torque quantities T.sub.1, T.sub.2, and T.sub.3 were such that T.sub.1 &gt;T.sub.2 &gt;T.sub.3 &gt;0, and corresponding domains T.sub.1 ', T.sub.2 ', and T.sub.3 ' of definition of the steering torque Ti, on which the output torque To was allowed to vary while ranging from zero to the maximum quantities T.sub.1, T.sub.2, and T.sub.3, respectively.
In this respect, in this power steering system, in which the dead zone Do had a constant band width independent of the vehicle speed, the proportion that the width of the dead zone Do occupied in that of such domain of the steering torque Ti was relatively large while the vehicle speed was in the low speed region, particularly when the speed was close to zero.
As a result, during an initial phase of the rotating operation of a steering wheel, the electric motor in the electromagnetic servo device was kept from being energized, so that the vehicle's driver had to exert relatively large steering forces to the steering wheel and thus the steering operation felt relatively heavy.
On the other hand, while the vehicle speed was maintained, in the high speed region, at a slightly lower speed than the aforesaid predetermined high speed, the steering torque Ti had a domain thereof narrowed to be somewhat smaller in width than the domain T.sub.3 ' given at the predetermined high speed, so that the proportion of the width of the dead zone Do to that of the domain of the steering torque Ti was relatively small.
As a result, in this power steering system, when it was operated, the electric motor of the electromagnetic servo device was likely to have on and off states thereof frequently repeated, whereas such repetition is unfavorable to the durability of the servo device itself as well as of the steering system.
The present invention has been achieved to effectively overcome such problems in conventional electromagnetic servo device for electric power steering systems for vehicles.